Great minds must think alike. In Dec I paid as far ahead as I could, I did a tester last week to see if I could still use this and received the cash back. Just rang again today and paid until late May next year. The offer was good for payments made until June I think so will ring back for another one then.I just rang BUPA and paid my yearly premium in advance to beat the price rise and got a : " You’ve just used your BUPA Amex Offer" email as as soon as the guy processed the payment.
I checked and I don't have any BUPA offers saved that is current, looking back there is an offere that expired Dec 2019, so will be interesting to see if I do get any credits!